Eddy-current-based contactless speed sensing of conductive surfaces

Contactless speed sensors are used in a broad area of applications in various industries such as machining, assembly lines and transportation. Commonly used technologies are based on optics (e.g. cameras, encoders), or electromagnetic effects (e.g. variable reluctance sensors, Hall sensors). However, these sensors require a non-uniform property of the moving target that can be detected. For example, variable reluctance sensors rely on the variation of the air gap, and Hall sensors require a magnetic field whose spatial distribution is dependent on the position of the mover. A clear disadvantage of all these systems is the fact that they require modifications on the target's geometry and/or magnetic properties, since they cannot measure the speed of a smooth body/surface. Moreover, some of them are sensitive to environmental conditions; e.g. dirt in case of optical encoders and high temperature in case of permanent magnets can render these systems ineffective. Therefore, an eddy-current-based contactless speed sensor is developed in this work for measuring the speed of smooth, electrically conductive surfaces in harsh operating conditions. An injection coil is used to induce eddy currents in the mover whose speed is to be detected, and two differentially wound pick-up coils are used to detect the speed-dependent deformation of the eddy-current field. Two-dimensional finite-element method (2-D FEM) is used for modeling the system and optimizing the sensor geometry as well as the injection frequency. Measurements taken from a prototype verify the validity of the design procedure and the analyzed speed-sensing concept.